Blood Testing for Colon Cancer Screening
Blood testing is NOT recommended for colorectal cancer screening in asymptomatic average-risk adults over 50. The established screening options are stool-based tests (FIT, gFOBT) or direct visualization methods (colonoscopy, flexible sigmoidoscopy, CT colonography)—blood tests are explicitly excluded from guideline recommendations. 1, 2
Why Blood Tests Are Not Recommended
The American College of Physicians explicitly states that clinicians should NOT use serum screening tests for colorectal cancer. 2 The U.S. Multi-Society Task Force specifically recommends against the Septin9 serum assay for screening due to insufficient evidence. 1, 3
Blood-based tests lack the evidence base that supports mortality reduction, which is the critical outcome for screening. All guideline-endorsed screening methods have demonstrated either direct mortality benefit through randomized trials (FIT, gFOBT, flexible sigmoidoscopy, colonoscopy) or strong indirect evidence through test characteristics. 1, 4
Recommended Screening Approach for Adults Over 50
First-Tier Screening Options
For average-risk adults aged 50-75 years, offer one of these evidence-based options:
- Annual fecal immunochemical test (FIT) - This is a first-tier screening option with proven mortality reduction. 1, 5, 3
- Colonoscopy every 10 years - This is the other first-tier option with strong evidence for mortality benefit. 1, 5, 3
- High-sensitivity guaiac-based fecal occult blood test (gFOBT) every 2 years - Supported by randomized controlled trials showing mortality reduction. 1, 2
- Flexible sigmoidoscopy every 10 years plus FIT every 2 years - This combination approach has mortality benefit from randomized trials. 1, 2
Second-Tier Options (When First-Tier Declined)
If patients decline colonoscopy and stool-based testing, consider:
- CT colonography every 5 years 1, 5, 3
- FIT-DNA (multitargeted stool DNA test) every 3 years - Note this has lower specificity than FIT alone, resulting in more false positives and unnecessary colonoscopies. 1, 5
- Flexible sigmoidoscopy alone every 5 years 1, 3
Age-Specific Considerations
Ages 50-75 Years
Begin screening at age 50 for average-risk individuals with strong evidence supporting substantial net benefit. 1, 5, 4 The American Cancer Society suggests considering age 45 for initiation, though this is a qualified recommendation with less certainty. 1
Ages 45-49 Years
Consider NOT screening this age group routinely. If screening is discussed, acknowledge the uncertainty around benefits and harms in this population. 2 African Americans may benefit from earlier screening at age 45 due to higher incidence rates. 1, 5, 3
Ages 76-85 Years
Individualize the decision based on:
- Prior screening history (never-screened individuals more likely to benefit) 1, 6
- Life expectancy exceeding 10 years 1, 7, 6, 5
- Overall health status and ability to tolerate treatment if cancer detected 6, 5
Age 85+ Years
Stop screening - The harms outweigh benefits in this age group regardless of prior screening history. 1, 6
Critical Implementation Points
All positive stool-based tests require follow-up colonoscopy for diagnostic evaluation—this is non-negotiable. 1, 5
Do NOT perform single-panel guaiac FOBT during digital rectal examination as this has unacceptably low sensitivity and is not a valid screening method. 5
Assess life expectancy before initiating screening - If less than 10 years due to comorbidities, screening is unlikely to provide benefit and should be avoided. 1, 7, 6, 5 The average time to prevent one CRC death is 10.3 years from screening initiation. 1
Common Pitfalls to Avoid
Do not use blood tests (including Septin9 DNA assay) as they lack evidence for mortality benefit and are not guideline-endorsed. 1, 3, 2
Do not continue screening past age 75 in patients with adequate prior screening history, as harms increasingly outweigh benefits with advancing age. 1, 6, 5
Do not screen patients who cannot tolerate cancer treatment if detected, including those with severe comorbidities substantially limiting life expectancy. 1, 6, 5
Ensure adequate bowel preparation for colonoscopy as this is critical for visualization and test accuracy. 1